1. Field of the Invention
The invention relates to apparatus for determining the location of passage of a projectile through a predetermined plane, from the shock or pressure waves produced by the projectile.
2. The Prior Art
A number of systems are known for determining the location at which a projectile passes through or strikes a predetermined plane.
For example, U.S. Pat. No. 3,022,076 to ZITO, teaches a system in which a projectile strikes the rigid surface of a target to generate shock waves which emanate from the point of impact on the target. Detectors are mounted near the edge of two or more sides of the target surface, and produce output signals in response to shock waves. By measuring the time differentials of the detector outputs, the exact location of the point of impact may be determined by converting the time relationships into distance measurements.
British Pat. No. 969,929, published Sept. 16, 1964, also teaches a target having transducers mounted at the edges of a rigid target material, the output of the transducer indicating impact by a projectile. A similar system is taught in U.S. Pat. No. 3,857,022 to REBANE et al, in which transducers attached to three sides of a screen convert acoustic signals into electric signals from which the X, Y coordinates of the point of impact may be determined. Further, comparable systems of this type are taught in U.S. Pat. No. 3,627,323 to BOZICH et al, and U.S. Pat. No. 3,707,699 to SANCTUARY.
The U.S. Pat. No. 3,678,495 to GILBERT discloses a target system in which transducers are spaced evenly around a rigid metal target. Acoustic waves developed in the target upon impact of a projectile are detected by transducers. The output of the transducers enables computation of the location of the point of impact.
Systems of the type described above have a number of disadvantages for use in marksmanship training. All detect shock waves transmitted through a rigid target member and, when such a member is used in marksmanship training with live ammunition, the target member must be frequently replaced or repaired. Those systems which do not require transducers along more than one edge of the target area, such as in U.K. Pat. No. 969,929, are not particularly accurate, since they can only indicate that a projectile has struck a given target area and cannot indicate the precise location of the point of impact. Those systems which can determine the point of impact require transducers to be positioned along at least two edges of the target area. This presents a problem when using live ammunition on a firing range because transducers positioned along the side or top edges of the target surface may be struck by the ammunition, causing damage to the target system.
Airborne shockwaves are detected by other types of systems. For example, U.S. Pat. No. 2,535,255 to BARNES et al, teaches a system for locating the source of sound waves, in which two microphones are spaced apart along a base line. The time interval between arrival of sound at the microphones is detected and, from that, partial information about the origin of the sound source may be derived.
U.S. Pat. No. 3,489,413 to GRODER et al, teaches a system in which a plurality of bi-directional transducers is arranged to monitor target zones and to indicate through which zone a projectile has passed.
U.S. Pat. No. 2,925,582 to MATTEI et al, teaches a target system in which microphones are placed on a circular crown around the target. The transducer output signals are employed to deflect the beam of a cathode ray tube to indicate the location at which the projectile passes through the target plane.
U.S. Pat. No. 3,985,024 to HORAK, discloses a further transducer array for determining the location of an acoustic source.
U.S. Pat. No. 3,778,059 to ROHRBAUGH et al, discloses further an automatic target system responsive to the airborne acoustic shock wave produced by a projectile. Mutually perpendicular elongated acoustic energy conductors are located adjacent the perimeter of the target and have one or more acoustic transducers attached to an end or ends thereof. Electrical signals are produced by the acoustic transducers in response to detected shock waves, and appropriate circuitry determines the location of each "hit" on the target in rectangular coordinates.
As with the target systems which detect target-transmitted shock waves, the systems detecting airborne shock waves also generally require transducers to be spaced apart in an array which is not easily protectable in a target range. It would be preferable to have a system in which the transducers, or detectors, are located only along the bottom edge of the target, where they may be easily protected by means of an earth embankment or the like. Transducers, or detectors, positioned along the side or top edges of the target plane are not desirable due to the likelihood of damage being caused by badly aimed rounds.